Examples of the present invention relate generally to systems for performing molecular analysis, such as surface-enhanced Raman spectroscopy (SERS), enhanced fluorescence, enhanced luminescence, and plasmonic sensing, among other systems.
With specific regard to SERS, Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in molecular systems. In a Raman spectroscopic experiment, an approximately monochromatic beam of light of a particular wavelength range passes through a sample of molecules and a spectrum of scattered light is emitted. The spectrum of wavelengths emitted from the molecule is called a “Raman spectrum” and the emitted light is called “Raman scattered light.” A Raman spectrum can reveal electronic, vibrational, and rotational energy levels of a molecule. Different molecules produce different Raman spectra that can be used like fingerprints to identify molecules and even to determine the structure of molecules.
Raman spectroscopy is used to study the transitions between molecular energy states when photons interact with molecules, which results in the energy of the scattered photons being shifted. The Raman scattering of a molecule can be seen as two processes. The molecule, which is at a certain energy state, is first excited into another (either virtual or real) energy state by the incident photons, which is ordinarily in the optical frequency domain. The excited molecule then radiates as a dipole source under the influence of the environment in which it sits, at a frequency that may be lower (i.e., Stokes scattering) or that may be higher anti-Stokes scattering) compared to the excitation photons. The Raman spectrum of different molecules or species (such as virus encapsulations) has characteristic peaks that can be used to identify the species. Accordingly, Raman spectroscopy is a useful technique in a variety of chemical or biological sensing and identification applications. However, the intrinsic Raman scattering process is very inefficient, and rough metal surfaces, various types of nano-antennas, as well as waveguiding structures have been used to enhance the Raman scattering processes (i.e., the excitation and/or radiation processes described above).
The Raman scattered light generated by molecules or species adsorbed on or within a few nanometers of a structured metal surface can be 103 to 1014 times greater than the Raman scattered light generated by the same species in solution or in the gas phase. This scattering cross section amplification process is called surface-enhanced Raman spectroscopy (“SERS”). In recent years, SERS has emerged as a routine and powerful tool for investigating molecular structures and characterizing interfacial and thin-film systems, even enabling single-molecule detection. Engineers, physicists, and chemists continue to seek improvements in systems and methods for performing SERS.